In conventional positioning devices, for example, head positioning devices in magnetic disk devices, there is a problem that it is difficult to perform high-speed positioning with a common characteristic to all target positions and in all kinds of devices since a transient response in settling a head position is disturbed by various reasons: 1) a conversion gain (torque constant) from control input to torque generation varies with time due to heating of a motor used for positioning control; 2) a torque constant varies depending on a place to be positioned, i.e., an outer track, center track or inner track of a disk; 3) a production error occurs in fabrication of disk devices etc. Meanwhile, recently, generally employed in magnetic disk devices is the so-called `sector servo scheme` in which a head position detection signal is written on each disk surface. The head position is controlled using a digital servo based on the detected signal. The proportion of the number of head position detection signals is however decreased due to a demand for increase in data capacity. Therefore, the sampling rate of digital servo-control devices trends to be reduced. Thus, the above problem becomes vary serious.
Various methods to solve such problem have been suggested. A simplest and typical method is to widen a feedback servo control band as long as possible. In the other methods, the internal state of a digital servo device is reset to reduce an overshoot or undershoot when settling starts in positioning the control system (see, e.g., Japanese patent application laid-open Nos. 3-288913, 4-335272), a corrective control input is learned and renewed depending on a speed follow-up error to enhance the accuracy of following-up to a reference speed track to improve the transient characteristic in speed control system (see, e.g., Japanese patent application laid-open No. 5-16941).
However, in the conventional sector servo manner magnetic disk device, it is difficult to widen the feedback servo control band since the sampling time of the servo system cannot be shortened. Thus, it is substantially impossible to perform high-speed positioning with a common characteristic to all target positions and in all kinds of devices.
In the method wherein the internal state of a digital servo device is reset to reduce overshoot or undershoot when settling starts in positioning the control system, it is necessary to know in advance the characteristic (model) of the controlled system. Thus, when all actual controlled system is varied with time, it cannot normally operate except unless an additional operation such as determining the characteristic again is conducted.
In the method of applying learning control to speed control, since digital control is not considered and the renewal of the corrective learning input is simultaneously conducted with the output of the trial at the next time, there is a problem that an excessive spike-shaped learning input result occurs when the sampling time is lengthened. In particular, this problem is significant in positioning control system. Therefore, the conventional devices that the renewal of corrective learning input is simultaneously conducted with the output of the trial at the next time, even if the sampling time is short, cannot be applied to positioning control.